The Mouse Genetics Core (MGC) will provide a variety of experimental services as well as training and advice pertaining to the goals of the investigators in this Program Project grant application. The experimental services provided include advice and training in gene isolation and characterization in Ipreparation for the construction of gene targeting vectors for use in embryonic stem (ES) cells. The MGC will assist in designing gene targeting strategies and screening approaches, as well as assist with gene transfer into ES cells by electroporation. These gene targeting approaches can include production of null alleles by classical or conditional mutagenesis approaches, 'knock-in' mutations, engineering of hypomorphic alleles, and strategies to mark targeted cells with fluorescent or luminescent gene products. Adequate provision of various essential reagents will also be a responsibility of the MGC. The MGC will maintain and make available gene targeting DNAS vectors, and totipotent ES cells from both 129 and C57BL/6 strains for use by the project investigators. In addition, the MGC will derive and provide embryonic fibroblast feeder cells as required to facilitate ES cell cultures. Furthermore, the MGC will identify appropriate lots of fetal calf serum to support growth and maintenance of totipotent ES cells. Both for essential testing of ES cells, and for experimental needs of the project investigators, the MGC will routinely produce chimeric mice by microsurgical and microinjection techniques to insure that available lots of ES cells are capable of contributing to the germline of chimeric mice at high frequency. The MGC will breed chimeric mice to produce offspring heterozygous for the ES cell-derived genome, and will provide these animals to the project investigators for their studies. Assistance with mouse importation, exportation, and genotyping will also be accommodated by the MGC.